Help me figure out this 12AX7 Tube Preamp mess

Discussion in 'Amplifiers' started by wukong, Dec 1, 2017.

  1. wukong

    wukong New Member

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    Hi to all, today I arranged my bookcase and found a magical thing, as shown schematic.
    A few years ago I built a Tube guitar amp01 / pedal / preamp with a 12ax7 tube (and some other pin compatible tubes). I don't remember what schematic I used to make it and I wasn't skilled with electronics then but surprisingly sound does come out of it, and I really like the distortion when the tube is overdriven.
    [​IMG]
    It looks like I've made some whack decisions when building it:

    1. I have no idea how I arrived at the resistor values, as well as their wattage
    2. The capacitors don't look like they are the right type or values
    3. The heater connections look weird, whats with the grounded middle?
    4. The pots look like they are 1M each, Is that sane?
    5. The amp is really quiet, even turning all pots to maximum with line level input. It distorts slightly, but I was expecting it to be massively distorted (in a good way) when turned up to the max.
    6. Whats with the 100uF Electrolytic caps on the cathodes of the triode?
    7. Where should the ground be connected in this setup. I'm getting intermittent hum when I connect a guitar to the input, and speakers/amp to the output, but fine if I play music to it from my phone
    8. Why the two resistors around the output potentiometer (see Update below)
    I would love for someone who is familiar with tube amplifiers to look this over and help me work out right values and parts.

    Here is an album with couple of pictures of the insides, Its not pretty.

    Update:
    Crud, I forgot about the two resistors across the output potentiometer. From the 2nd picture from the bottom of that album, the resistor across output and ground is 100k and one across capacitor and output is 1M
    Sorry to much question.
    Thanks all!
     
  2. elvis

    elvis Hamfisted String Banger

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  3. elvis

    elvis Hamfisted String Banger

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    Incidentally, you're stuck with electrolytic caps when they are more than a few microFarads. Any other cap type would be physically enormous.
     
  4. Dusty Chalk

    Dusty Chalk alberngruppenf├╝hrer

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    You say that like it's a bad thing.
     
  5. elvis

    elvis Hamfisted String Banger

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    Makes me feel inadequate.
     
    Dusty Chalk likes this.

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